1. Field of the Invention
This invention generally relates to a process and system for removing sulfur oxides such as sulfur dioxide from gases by scrubbing with an aqueous sodium-base solution, where the gases result from a combustion process employing a fuel which has appreciable chlorine content.
2. State of the Art
To protect the environment and public health, it has been established that industry should limit the concentration of sulfur oxides released to the atmosphere as the result of combustion processes. Control of sulfur oxide emissions is, however, difficult to accomplish under circumstances where it is necessary that the control system operate dependably enough to not interfere with the combustion process itself. For example, electric power generating plants are obligated to control sulfur oxide emissions from their coal-fired boilers, but must do so in a manner which will not interrupt power generation.
A number of techniques have been proposed to remove sulfur dioxide from combustion gases. One well known technique is the so-called "limestone scrubbing" process wherein combustion gases are scrubbed with an aqueous slurry containing calcium in solution derived from the partial dissolving of limestone. When combustion gases containing sulfur oxides are scrubbed with such a slurry, the calcium ions ultimately react with the sulfur oxides to some degree in the scrubber to form relatively insoluble products, principally calcium sulfate and sulfite, which can thereafter be separated from the scrubbing slurry by filtration or sedimentation techniques or combinations of both. A disadvantage of such limestone scrubbing systems is the risk, especially during periods of equipment malfunction, of inadvertently forming large deposits of insoluble materials in the scrubbing equipment. Such deposits, called scale, can irreparably foul the scrubbing system and associated piping.
To avoid the aforementioned risks, workers in the art have proposed the use of sodium-base scrubbing solutions. In these systems, combustion gases are scrubbed with solutions containing sodium hydroxide and/or sodium sulfite. These solutions react with sulfur dioxide to form sodium bisulfite, a soluble compound which does not cause scale. A number of processes have been proposed to reactivate the scrubbing solutions by treatment with an inexpensive alkali substance such as lime or limestone to recover the relatively more expensive sodium values and to convert the collected sulfur oxides to a solid form for ready disposal. In such regeneration processes, solid calcium sulfite and sulfate particles are formed and, at the same time, the sodium sulfite and/or hydroxide concentration of the solution is increased at the expense of sodium bisulfite. Typically, the calcium-bearing solid particles are separated from the regenerated solution and discarded.
An early sodium-base scrubbing process is suggested in U.S. Pat. No. 1,271,899 to Howard; according to that patent, a sodium bisulfite solution which resulted from the scrubbing of flue gas is reacted with lime to precipitate calcium sulfite and to regenerate sodium hydroxide. Anothe process of this type is suggested in U.S. Pat. No. 3,697,033; according to that patent, a solution of sodium sulfite and bisulfite resulting from the scrubbing of combustion gases is sequentially treated with limestone and then lime in order to precipitate calcium sulfite and sulfate and to regenerate the sodium-base scrubbing solution. Still another example of a sodium-base scrubbing process can be found in U.S. Pat. No. 3,772,532 to Dahlstrom and Cornell. Yet another example can be found in U.S. Pat. No. 3,911,084; the process proposed in that patent includes a pretreatment step wherein gases are cooled to below 500.degree. F. and the bulk of larger particulates are removed.